So, you need to monitor the levels of several tanks from a remote point. What is the best way to do this, and where should you begin? For many situations, daisy chaining the sensors—for example, in an RS485 Modbus network—is a simple, reliable, accurate, and cost effective way to simultaneously monitor the levels in several tanks. The best way to get started is by following a few basic installation practices.
Step 1: Get The Right Sensor and System
We’ve said this several times in different ways over the past couple months: make sure your sensors make sense for your application. This applies to your communication system as well. You can use the very best installation practices, but if your sensors or communication system aren’t right for the job, none of it will matter. So, before you buy anything, make sure you have accounted for:
- Hazardous Locations
- Corrosive, Damp, or Dusty Environments
- Sensor Mounting
- Sensor and Communication Power
- Sensor Signal Type
Step 2: Hook Up!
Since we’re primarily interested in the daisy chain installation, we’ll assume that the physical installation of your sensors was a walk in the park. No problems at all. Wasn’t that easy?
Right. Now to the fun part: wiring your sensors. Daisy chain wiring has several advantages over individual home run wiring: less cable, shorter runs, doesn’t require larger sizes of conduit. But that doesn’t mean daisy chain installation is mindless. Good wiring practices still apply.
- Always use properly shielded cables that are the appropriate gauge for you application.
- Never route low voltage (24/48 VDC) or communication/control wiring adjacent to AC cable (120 VAC or above). A good rule of thumb is to keep them at least 6 inches apart if they must be run parallel to each other, and use RGS conduit for the LV/signal cables. NEC Article 725 is a good reference.
- Never coil excess cable or lay it on top of AC power conduit. Any signal vs power crossings should be perpendicular to minimize electromagnetic interference (EMI).
- Make sure electrical grounds are terminated properly to prevent ground loops or potential ground shorts that can adversely affect instrument functionality.
- If possible install your instrumentation away from other machinery or voltage sources that produce large currents.
Step 3: Don’t Get Struck By Lightning!
“Dear Lightning: Please stay away from this installation. Kind Regards, Every Maintenance Crew Everywhere.”
Since lightning has yet to prove it can read, an open letter is probably not enough to protect your installation. So what can you do? If your tanks, sensors, wiring, etc. are all installed inside a building, and that building is appropriately protected, then everything should be good. But if the installation is completely open to the elements, make sure the structure of each tank is adequately protected. Also, ask the manufacturer of your sensor is it has built-in lightning protection.
Step 4: Power Up!
Once everything is correctly installed, it’s time to power up your system. Be sure to follow any instructions about sequential instrument addition to your communication system. For example, Modbus-RTU systems require that each device be added to the system one at a time to prevent device address confusion. Being aware of little details like this before you start will greatly increase the likelihood of a successful installation.
Have questions about an upcoming installation? Having difficulties troubleshooting an existing installation? Give our helpful Measurement Experts a call, shoot us an email, or even live chat with us. Whether your signals are crossed or your sensors are incensed, we can help you get hooked up and powered up right.